In the field of utilization of solar energy, the photothermal conversion process is of prime importance. This process enables for example thermal energy to be produced for domestic heating or air conditioning, and enables hot water (pressurized) or steam to be obtained for the production of mechanical energy or for various other industrial processes etc. Photothermal conversion may be defined as the absorption of an incident electromagnetic radiation (such as solar radiation) by an absorbent collecting surface, with subsequent heating of this collecting surface, which in its turn heats a determined heat exchanger fluid or load. In general one seeks to obtain the highest possible operating temperature for a given incident radiation power per unit of converter surface (the intensity of solar radiation is notably limited and depends also on the hour of the day, the meteorological conditions etc. With regard to direct solar radiation, optical concentration may be used by means of collecting mirror or lens systems). To obtain a high operating temperature, effective thermal insulation of the two faces of the converter is necessary in addition to high incident radiation intensity. Thermal insulation of the face exposed to the incident radiation however is difficult to effect, because this face has at one and the same time to absorb the incident radiation and refrain from emitting thermal infrared, so that it must be made absorbent with regard to incident radiation and reflecting with regard to emitted radiation.
In any photothermal converter, three well known thermal loss processes must be considered: losses by thermal infrared radiation omission, cooling by convection to the interior of the gaseous volume separating the converter from its immediate surroundings, and losses by thermal conduction.
In recent years numerous methods have been proposed for minimizing the total of said thermal losses, and consequently increasing the photothermal conversion efficiency. To reduce radiation losses it has been proposed for example to use surfaces which are selective to radiation. These surfaces, of low thermal emissivity, in particular allow practically complete absorption of the incident solar radiation, while strongly reducing losses by infrared radiation from the converter. However, the presence of such surfaces contributes to strongly increasing the cost of the converters, and poses long term stability problems. Three main methods are known for reducing convection losses. The first consists of stacking several transparent cover plates above the converter so as to confine the convection mechanism to within volumes of lower temperature differences. However this plate stacking contributes to increasing losses by reflection of the incident radiation, and results in an increase of the weight and cost of the device. The second method consists of filling the space above the converter with a gas having a thermal conductivity lower than air, while the third method consists of completely evacuating this air space. However the second method enables only a partial reduction of the conduction/convection losses to be obtained. The third method tends to be costly, as it requires the presence of absolutely airtight enclosures which have a low degassing rate. The construction of long life airtight enclosures is moreover difficult, bearing in mind that these enclosures are subjected to considerable thermal cycles (variations in the pressure of the thermal insulation gas) and to other atmospheric attack, undermining in particular its airtightness. Finally, in the particular case of application to solar collectors, the appearance of any crack in the covering glass (hail, falling branch, thrown stones ...) has the inevitable effect of making these collectors unworkable by causing escape of the low thermal conductivity gas.
For collecting the solar energy it has also been proposed to use a honeycomb structure resting on a conventional absorbing surface. This structure is preferably made of fine glass plates or tubes, which may typically have a height of 6 to 25 cm, a diameter of 0.5 to 1.5 cm and a thickness of 0.2 to 0.3 mm. Such a structure has the advantage of serving as a light guide for the incident solar radiation, which undergoes a multiplicity of reflections and refractions before being absorbed by the actual converter. Where the height-diameter ratio is sufficiently high, such a structure may also serve as a thermal barrier for the re-emitted infrared radiation, which is compelled to follow a diffusion process before being able to reach the exterior, with consequent reduction in radiation losses. However, such a structure does not give optimum reduction in convective air movements, especially where there is a large temperature difference between the converter and its immediate surroundings, and/or where the converter is inclined. In this respect, a limiting air layer of small thickness (about 1 mm) and of unstable buoyancy notably forms above a horizontal heated surface, and convective filament-type movements develop from this limiting layer which mix by convection with the air layers situated further above the hot surface. It is equally known that this instability, in the case of an inclined cellular structure, takes the form of regular circulation within each cell.
As these constituent cells of the honeycomb structure have lateral dimensions which are considerably greater than the characteristic diameter of these filament-type convective movements (or than the thickness of the movements assuming the form of a regular circulation-ratio greater than 2), it follows that such a structure is not capable of suppressing the convective air movements in an optimum manner, and consequently preventing the cooling of the converter by convection. Moreover, this honeycomb structure is relatively thick, given the height (6 to 25 cm) of the cells, so that a solar collector equipped with such a structure risks being too heavy and too bulky for the majority of applications. The need to use a considerable quantity of glass for this structure (of the order of 12 to 60 kg/m.sup.2 of converter) further leads to a total weight and prices which are hardly competitive. Moreover, this large mass of glass gives rise to very high thermal time constants, so that one or several hours of continuous exposure to solar radiation are necessary before such a collector attains its working temperature.
The object of the present invention is to remedy the various aforementioned disadvantages, by proposing a photothermal converter of high conversion efficiency and economical cost.